Blog editor's note: Despite that McClatchy Newspapers are not considered part of the foreign policy "elite" media, they frequently do very good independent reporting. In my view, this is because McClatchy last year bought the Knight-Ridder chain, which for whatever reasons had established a first-rate group of journalists covering everything from the Defense Department to the run-up to the 2003 Iraq war. Indeed, KR was one of the few mainstream news organizations to seriously question the Administration's claims that Iraq had WMDs.

WASHINGTON - Sunni Muslim insurgents remain by far the biggest threat to American troops in Iraq, despite recent U.S. claims that Iran is providing Shiite Muslim militia groups with a new type of roadside bomb, a review of American casualty reports shows.

While U.S. military officials have held briefings to publicize their concerns about the potent bombs known as explosively formed projectiles (EFPs) or penetrators, casualty reports suggest that such weapons in the hands of Shiite militias are responsible for a relatively small number of American deaths.

U.S. officials have said that attacks with such weapons increased 150 percent in the past year. But a review of bombings by location shows that less than 10 percent of attacks that killed at least two American service members in the past 14 months were in areas where Shiite militias are dominant.

Those reports show that fewer than half the bomb attacks on heavily armored U.S. vehicles such as Abrams tanks and Bradley Fighting Vehicles were in areas where Shiite militias dominate.

While it's difficult to know which armed group planted a bomb, analysts say the casualty numbers show that U.S. officials are exaggerating the importance of EFPs, which military officials say have been used only by Shiites.

"There were relatively few American deaths from explosively formed penetrators until recently, but you can say the same thing about attacks on helicopters or chlorine attacks," said Loren Thompson, a defense analyst with the Lexington Institute, a policy research group in Arlington, Va. "The fact of the matter is that the insurgents, both Sunni and Shiite, are becoming a lot more sophisticated in their tactics. Explosively formed penetrators are only one part of that, and they are not a particularly important part."